The phrase "proper sequel" refers to a follow-up or continuation of a story or event that is done in the correct or expected way, keeping the original essence and quality intact. Full definition
I don't think it means they aren't working on proper sequels to any of those games, either. (waypoint.vice.com)
Utilitarian games can't get proper sequels unless a new use or an improved method becomes available. (nintendoworldreport.com)
I think if this is going to happen, though, it has to happen sooner in the Switch's life than later (if nothing else, to keep hopes up for proper sequels to this and Mario Kart 8), so this would be the event to at least announce it — and perhaps reassure people those last few Smash - themed amiibo are still on the way, probably to launch alongside the new version. (poisonmushroom.org)